The present invention relates to an improved lens design. More specifically, the present invention relates to an improved multifocal lens using one or more aspheric surfaces for vision correction.
A spherical lens has a front and back surface with each surface having a constant radius of curvature. The focal power of the spherical lens is also constant. As you move along the lens in a radial direction from the center point to the periphery, the optical power of the spherical lens does not change except for smaller order effects due to optical aberration.
An aspheric lens on the other hand has a non-constant radius of curvature on one or both of its front and back surfaces. The focal power of the aspheric lens changes as you move along the radius of the lens. This feature is the basis for a multifocal vision correcting lens.
The degree to which an aspheric lens departs from a spherical lens is measured by the eccentricity parameter e. If e=0, the lens has a spherical surface. If e=1 the lens has a parabolic surface; if e greater than 1 the lens has a hyperbolic surface, and if e less than 1 the lens has an elliptical surface.
One use of the aspheric lens, particularly a contact lens, is to correct presbyopia (a vision condition associated with age). Over time the presbyopic patient loses visual accommodation (i.e., the ability of the eye to change optical power in order to adjust focus for different viewing distances) such that objects at near or intermediate viewing distances are not seen clearly without the aid of a near power lens. The aspheric lens compensates for presbyopia by providing a range of optical power including that required for far, near, and intermediate viewing distances. Generally, by increasing the eccentricity e, the range of optical power provided by the aspheric lens increases such that the value of e may in principle be adjusted for early or advanced presbyopia. However, there appears to be a maximum eccentricity value which is useful. With current designs with e values below approximately 0.8, additional near power of up to approximately +1.50 D is possible. This is suitable for early to moderate presbyopia. For moderate to advanced presbyopia +1.50 to +2.50 D (or more) of additional near power are required. However, if the eccentricity e is increased above approximately 0.8 to provide this increased level of additional near power, it is found that the quality of distance vision becomes so compromised as to be unacceptable to most patients.
In U.S. Pat. No. 4,704,016, a multifocal contact lens is disclosed. The major viewing area of the lens is divided into a multiplicity of near and distant vision viewing zones. The wearer is able to simultaneously look through at least two zones of different power. One way of creating the zones is to form a series of concentric rings using a lathe. The annular area of the lens is cut alternately for distant and for near vision correction. The eccentricity of the surface is varied in dependence on the radius but there is no dependence on the equatorial angle xcfx86. Another technique disclosed in the patent is to incorporate segments of material having a different refractive index from that of the body of the lens. The eccentricity of these lenses is also independent of the equatorial angle xcfx86. These lenses do not solve the problem of channeling too much light into the near vision portion of the lens.
U.S. Pat. No. 4,898,461 discloses a lens similar to U.S. Pat. No. 4,704,016. Like the foregoing disclosure the lens has a plurality of alternating focal power zones. Here, the focal power varies continuously in the radial direction within each zone and in the transition area between each zone. The eccentricity of these lenses is independent of the equatorial angle xcfx86. These lenses also do not solve the problem of channeling too much light into the near vision portion of the lens.
Another contact lens design has been proposed for achieving near and distant vision correction known as the translating design. Translating designs attempt to exploit the fact that when a wearer looks down to read, a contact lens rides up on the wearer""s cornea. Translating designs thus attempt to place an optical zone with the distance power over the pupil of the eye when the patient is looking straight ahead and an optical zone with the near power over the pupil when the patient is looking down to read. However, sufficient and reliable translation has not been achieved to make the lens satisfactory in most applications. Also, the comfort of translating designs is often unacceptable to many patients.
There is a need for an improved multifocal lens which eliminates some or all of these problems found in the prior art lens designs.
The present invention provides, according to a first aspect, a contact lens having a front surface, a back surface and an apex. The lens defines a series of adjacent points at a fixed distance from the apex. The series of adjacent points on the lens having a continuously varying power, the series of adjacent points extending across an arc of at least 120xc2x0.
According to another aspect of the invention, the contact lens includes a front surface and a back surface. One of the front surface and the back surface is an aspheric surface wherein the eccentricity varies continuously as a function of the angle xcfx86.
According to yet another aspect of the invention, a bottom portion of the lens has an eccentricity that varies continuously as a function of the angle xcfx86 and a top portion of the lens has a substantially constant eccentricity as a function of the angle xcfx86.
According to a further aspect of the invention, the lens includes two side portions that have an eccentricity that varies continuously as a function of the angle xcfx86 and top and bottom portions that have a substantially constant eccentricity as a function of the angle xcfx86.
The lens of the present invention has several advantages over prior lenses including an enhanced visual acuity at near and distance powers. In addition, the present invention overcomes the add power problem of previous aspheric lenses while retaining the advantages of an aspheric lens, i.e., to provide an intermediate vision capability.
These and other aspects and features of the invention will be further understood when considered in conjunction with the following detailed description of the embodiments of the invention.